The present invention relates to a process for the preparation of isocyanates and/or derivatives thereof. Isocyanates are compounds containing at least one group having the structure --N.dbd.C.dbd.O. They constitute very important starting materials for chemical processes, especially in the manufacture of polyurethanes which find wide application in the synthetic foam industry.
Isocyanates are normally prepared via the so-called "phosgene route": an appropriate amine is reacted with phosgene (COCl.sub.2) to form the required isocyanate. That process, however, suffers from the severe drawback that phosgene is a chemical which is highly toxic.
Numerous attempts have been made over the years to find alternative routes for the preparation of isocyanates, especially the commercially interesting ones such as toluene diisocyanate (TDI) and methylene-4, 4'-diphenyldiisocyanate (MDI) via a "phosgene-free" route. Reference is made in this respect to "Recent Advances in Isocyanate Chemistry" by S. Ozaki (Chem. Rev. 72 (1972) 457-496). For instance, much emphasis has been laid on the reduction of aromatic nitro compounds with carbon monoxide in the presence of a lower alkanol and a selenium containing catalyst. The carbamates obtained can be converted easily into the corresponding isocyanates. However, a seemingly unavoidable disadvantage of that process is the presence of small but definite amounts of selenium (compounds) in the product so that even more research is devoted to solve the selenium problem.
Although isocyanates can be obtained under very severe reaction conditions (e.g., the dehydrogenative composition of formanilide and oxanilide at high temperatures, i.e., 750.degree. C.-1000.degree. C. under reduced pressure in the presence of a Pd/Ni MgO-catalyst (see Z. Chem., (1974), Part 5, page 192)) no general process has been suggested thus far which generates isocyanates and/or the corresponding carbamates under mild reaction conditions and which is not impaired by catalyst-removal problems.
It has now surprisingly been found that isocyanates and/or derivatives thereof can be obtained under mild reaction conditions and without substantial work-up problems by the oxidative conversion of starting materials possessing the structure --NH--CO--CO--X in the molecule. It is the interesting feature of the present invention that the choice of oxalic acid derivatives as starting materials in the process according to the present invention enables the use of simple and well-known oxidation techniques. Moreover, a great number of different oxalic acid derivatives can be used as starting materials.